Ion implantation is a standard technique for introducing conductivity-altering impurities into a workpiece. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the workpiece. The energetic ions in the beam penetrate into the bulk of the workpiece material and are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity.
Solar cells are one example of a device that uses silicon workpieces. Any reduced cost to the manufacture or production of high-performance solar cells or any efficiency improvement to high-performance solar cells would have a positive impact on the implementation of solar cells worldwide. This will enable the wider availability of this clean energy technology.
In fabricating a solar cell, two factors must be considered. The first factor is series resistance (Rs), or the total resistance of the solar cell material. Rs limits the fill factor, or the ratio of the maximum power point divided by the product of the open circuit voltage (VOC) and the short circuit current (ISC). As Rs increases, the voltage drop between the junction voltage and the terminal voltage becomes greater for the same flow of current. This results in a significant decrease in the terminal voltage and a slight reduction in ISC. Very high values of Rs also produce a significant reduction in ISC. In such regimes, the Rs dominates and the behavior of the solar cell resembles that of a resistor. Thus, if VOC and/or ISC decrease, then the cell efficiency decreases as well. This decrease may be a linear function in one instance.
The second factor is photon conversion efficiency, which limits ISC. If the front surface of a solar cell is doped at a high level, series resistance will be reduced but recombination loss of the charge carriers increases. This recombination occurs due to interstitial dopants that are not incorporated into the crystal lattice. These dopant sites become recombination centers. This phenomenon is called Shockley-Read-Hall recombination. A solution that reduces recombination loss is to elevate doping levels only under the front surface contacts of the solar cell. This technique is known as a selective emitter.
One method to form a selective emitter in a solar cell is to perform a high-dose implant selectively in a region where the metal contacts or other conductors will eventually be formed. This requires either an expensive photolithography step or the use of a shadow or stencil mask to perform a selective or patterned implant. If a shadow or stencil mask is used, it must be carefully aligned to the desired implant areas. This may require an accuracy of approximately 10-20 μm for current solar cell designs. Accordingly, there is a need in the art for an improved method of selectively doping or modifying workpieces.